Combustion of solid and many liquid fuels typically results in the generation of several effluent streams, which must comply with increasingly stringent requirements to prevent the discharge of noxious species. The increasing cost of energy provides incentives to utilize low cost fuels which often produce higher levels of pollutants, particularly in the ash residues from the combustion. In addition, there is increasing need to combust waste-derived materials, with or without energy recovery, as a method for disposing of this material. Combustion of such material can generate hazardous species which are incorporated into the ash. Included in this category are heavy metals such as cadmium and lead. It is well understood that these species tend to accumulate in the fly ash due to the volatile nature of their oxide and chloride compounds. Water-soluble salts of these metals are detrimental if they leach from the ash. Hence there is a problem because it is desired to reduce the amounts of pollutants emitted by combustion processes, while there is a trend for pollution to increase due to the nature of the fuel, and the need to dispose wastes.
It is also desirable to reduce the amount of NO.sub.x (including NO and NO.sub.2) in flue gas discharges from combustion systems. Commercial technologies which use urea or ammonia are often not suitable in processes combusting waste materials, due to the high chloride content in the waste. The chlorides react with residual NH.sub.3 in the exhaust gas, creating a high opacity plume.
Both physical and chemical treatment methods are known to stabilize soluble heavy metal species in ashes, and thereby prevent the pollutants from migrating out of the ash. Examples of these treatments include heating to extremely high temperatures, in the absence or presence of added reactants, to vitrify metal species into a chemically inert glass. Several chemical treatments have also been disclosed. In the Journal of Air Waste Management Association, E. F. Barth in his article "The SITE Demonstration of the CHEMFIX Solidification/Solidification Process at the Portable Equipment Salvage Company Site", describes the a process, in which soluble silicate reagents are added to waste materials for stabilization via several mechanisms. U.S. Pat. No. 4,737,356, assigned to Wheelabrator Environmental Systems, Inc., discloses the addition of water-soluble phosphates to separated combustion ashes to decrease the mobility of lead and cadmium. Behel, et al, in their article "Attenuation of Cd and Pb Solubility in Municipal Waste Incineration Ash", in Soil Sci. Plant Anal., Vol 17, No. 4, pp 385-392 (1986) discuss addition of lime, calcium carbonate, alkali phosphate salts, elemental sulfur, and various wet and dry wastes from coal-fired steam plants for stabilizing ashes from waste-to-energy facilities. Pending U.S. application Ser. No. (811-P-US04267) 07/757,359 also describes a process in which calcium-containing compounds are added to fly ash and the mixture is thereafter subjected to thermal treatment. In each of these cases, the treatment of the subject material, e.g. ash, occurs after generation and separation. Consequently, for these chemical treatments to be effective, the chemical agent must be intimately mixed with the ash. Since the ash is typically a dry powder or granular material, effective contacting is difficult and requires additional equipment, such as ball mills, blenders, or other solid/solid or liquid/solid contacting devices. There is no prior art on the addition of elemental phosphorus for ash stabilization.
Another technique for metals stabilization is described in pending U.S. application Ser. No. (811-P-US04689) 07/757,361 wherein fly ash obtained after lime scrubbing is subjected to thermal treatment.
Addition of red phosphorus to the flue gas of combustion processes for reducing the concentration of NO.sub.x therein has been proposed in Patent Application 32 38 424 of Hoechst AG published by the Federal Republic of Germany. Specifically, this document teaches that the intimate contacting of such flue gas with an oxidizing gas and a suspension of red phosphorus in an inert liquid, such as water, is effective to remove NO. The operability of this suggested technique is, at best, erratic and not adequately effective. Chang and Lee, in their paper entitled "ILBL PhosNOX Process for Combined Removal of SO.sub.2 and NO.sub.x from Flue Gas", presented at the 1991 Summer AICHE Summer National Meeting, discuss contacting an aqueous mixture or slurry of yellow phosphorus with a flue gas to remove NO.sub.x. The contacting is effected by introducing the yellow phosphorus into either the acid gas scrubber or prescrubber, which is shown to be located downstream of the particulate solids removal, i.e. the bag house or electrostatic precipitator (ESP). Both of these references are directed solely to the reduction or removal of NO.sub.x and do not address to problem of metals stabilization in ash.